The soil-plant relationship of nitrogen use in three tropical tree species.

We characterized and compared the use of nitrate (N-NO3-), in three recognized pioneer and secondary tropical species, analyzing their relationships with the availability of inorganic nitrogen in the soil of a riparian forest. We tested the hypothesis that pioneer species of the ecological succession (Cecropia glaziovi - Cgl) would be more responsive to N-NO3- available and would have high nitrate reductase activity (NRA) in leaves while secondary species (Inga marginata - Ima and Hymenaea courbaril - Hco) would have less responsiveness and would have lower levels of this enzyme on its leaves. We evaluated, tested and compared the NRA and N-NO3- content in leaves and xylem sap of each species. We measured N-NO3- and N-NH4+ content in the soil. The results for Cgl showed an N-NO3- content of xylem sap that met the demand of NRA and were correlated with inorganic soil nitrogen, showing the responsiveness of species to N-NO3-. Hco was verified as less responsive to assimilation and high N-NO3- content in leaves. The species Ima showed an intermediate behaviour, which suggests the possibility of use of other nitrogen forms. The ecophysiological behaviours of the plants were related to the seasonal variation and indicate that these plants use strategies with different responsiveness to the use of soil N-NO3-. In this sense and considering that these species are typically used in ecological restoration projects in Brazil, the knowledge of a strategy for nitrogen use specific to each of them can assist in choosing the species appropriate to the edaphic conditions of the environment. [ABSTRACT FROM AUTHOR]